Some internal-combustion engines use an exhaust-gas recirculation (EGR) system to limit nitrogen oxide (NOX) emissions and provide other advantages. In such engines, some of the exhaust gas released from a combustion chamber during an exhaust stroke is fed back into the combustion chamber during a subsequent intake stroke. By diluting intake air with exhaust gas, the EGR system lowers the peak combustion temperature in the combustion chamber, thereby reducing the rate of NOX formation therein. Further, EGR reduces the concentration of oxygen in the exhaust stream, which may increase the efficiency of some exhaust-aftertreatment devices, particularly those used in diesel engines.
An EGR system may include an EGR cooler—a heat exchanger configured to reduce the temperature of the exhaust gas before it re-enters the combustion chamber. Some EGR coolers comprise a flow tube through which heat from the exhaust is transferred to a coolant fluid, e.g. the engine coolant. With continued use, however, the EGR cooler may be subject to fouling. In particular, adherent residues accumulated inside the flow tube—products of incomplete combustion, soot, etc.—may reduce the efficiency of the EGR cooler by limiting heat transfer. More significant accumulation of adherent residues may restrict gas flow through the EGR system. Either or both of these conditions may lead to combustion-control difficulties, increased engine pumping work, and ultimately to increased NOX emissions.
One remedy for the problem of EGR cooler fouling is to periodically disassemble the EGR system and clean the flow tube mechanically, or with the aid of solvents, detergents, etc. The disadvantage here is increased maintenance cost for the engine system.
Another approach is described in U.S. Pat. No. 6,826,903. Here, degraded EGR cooler performance is detected when intake pressure falls below a predetermined value. When degraded EGR cooler performance is detected, the temperature inside the EGR cooler is increased by heating the exhaust gas to eliminate soot or unburned hydrocarbons by oxidation. A related approach is described in U.S. Pat. No. 7,284,544, wherein hot exhaust gas is used to burn off adherent residues accumulated elsewhere in the EGR system.
However, under some conditions, the high-temperature oxidation approaches cited above may use increased amounts of heat-resistant components, modified valve timings to provide exhaust gas heating, and additional electronically controlled valves—in coolant flow paths, for example. Alternatively, without such additional measures, cooler degradation may occur more readily due to increased oxidation at high-temperatures.
The inventors herein have recognized these disadvantages and have devised an approach to at least partially address EGR cooler fouling. Thus, one embodiment provides a method to dislodge an adherent residue in an EGR cooler, the EGR cooler installed in an engine. The method comprises flowing an exhaust gas through an adherent-residue bearing surface of the EGR cooler when a temperature of the adherent-residue bearing surface is below a dewpoint temperature of the exhaust gas, enabling water vapor from the exhaust gas to condense on the adherent-residue bearing surface, heating the adherent-residue bearing surface to above the dewpoint temperature of the exhaust gas, and enabling condensed water on the adherent-residue bearing surface to evaporate. Other embodiments provide methods to recirculate an exhaust gas from an exhaust port of an engine to an intake of the engine via an EGR cooler, and related EGR systems, for example.
Specifically, the inventors herein have recognized that water condensation at lower temperatures, such as during cold start operation, may interact with an adherent residue in an EGR cooler, and when the condensed water later evaporates, at least some of the adherent residue may be released, without requiring temperatures high enough to oxidize the adherent residue. However, in some embodiments, the above approach may be combined with high-temperature removal of adherent residue in the EGR cooler, if desired.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the Detailed Description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the Detailed Description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.